The problems which arise are due to the frame/field rate differing between cine, NTSC and PAL video signals. These are NTSC 525 lines/60 fields with a frame rate of 30 per second, PAL 625 lines/50 fields with a frame rate of 25 per second and cinefilm having a film frame rate of 24 per second.
When cinefilm is taken as the source, that is the direct source for conversion to PAL video signals, there are two known techniques.
The first conversion technique involves direct conversion from cinefilm to
video signals. This technique uses a tele-cine machine to run the cinefilm at an increased speed, in fact at an increased rate which is 4% faster such that the PAL video signal has a 25 frame per second rate of cinefilm. This direct conversion works very well when the original source cinefilm is available.
The second technique which is employed for such conversion involves the intermediate step of converting the cinefilm to an NTSC video signal and then converting that video signal to a PAL video signal. This technique involves a conversion from cinefilm to NTSC video signal which uses what is defined as the 3:2 pull down technique. The technique treats the first and second frames of a pair in the following manner. The first frame is repeated to provide three television fields, and the second frame is repeated to provide two television fields, thus the NTSC video signal comprises 60 television fields per second or a frame rate of 30 television fields per second as compared with the original cinefilm frame rate of 24 frames per second.
The present invention addresses the deterioration in viewing quality of PAL transmission of pictures converted by the second of these techniques, particularly in the case where the editing of the film has been affected by NTSC video editing techniques.
The resulting product is considerably inferior by comparison with the product produced by direct tele-cine conversion from cinefilm to PAL video signals.
The reasons for the deterioriation in quality are:
1. the conversion process introduces unavoidable degradations
2. the conversion process involves temporal interpolation which means that the PAL video output signal (625 lines/50 fields) consists, in general, of a mix of neighbouring NTSC video signal input fields (525 lines/60 fields), these neighbouring fields sometimes containing information derived from the same cine-frame and sometimes derived from two sequential cine-frames. This leads to the blurring of moving images when the final product is viewed on a receiver.
3. the overall process of temporal interpolation is attempting to produce a
format with 25 output frames per second from a source (originally cinefilm) which was transferred to NTSC format by the 3:2 pull down technique at 24 frames per second. Despite the use of temporal interpolation, the difference frequency is visible as a judder effect occurring at the different frequency of 1 Hz during camera panning over a suitable interval.
4. the original conversion from cinefilm to NTSC format introduces a 12 Hz judder characteristic which is embedded in the NTSC video signal (525 lines/60 fields). Whilst this 12 Hz judder characteristic is acceptable for NTSC transmission, this characteristic, when it is subsequently converted to PAL video signals, further detracts from the quality of the converted PAL transmission.